Originally Posted by alien
No it ain't. Show me where it says that.
Please don't immediately naysay without doing your own fact checking and/or basic search.
First off, as a chemical engineer and former hazardous waste lab pack chemist, we encountered JB Weld and other isocyanates all of the time. Unmixed and uncured they are considered a flammable solid, class 4.1. However, that is because when mixed, the resin and hardeners actually undergo an exothermic reaction and release heat in small quantities. That being said, once cured and all solvents have evaporated (15-24 hrs of set time), the now uniform solid gray putty is virtually an inert piece of plastic. Unmixed JB weld and other isocyanates were always packed separately from each other (i.e. resin in one bucket, hardener in the other), but cured and hardened JB Weld and other isocyanates were tossed into the drums with all of the other non-hazardous chemicals (i.e. talc, sodium chloride and other salts, and tons of other chemicals). As an aside, I never once got a ticket for improper packaging of materials and I also never screwed up a drum inventory or manifest.
Read the JB Weld FAQ page and see what it says. It states that "When fully cured, J-B Weld is non-toxic. However, we do not recommend consuming the product." So don't go eating any. Also, it states that "Original J-B Weld can withstand a constant temperature of 500ºF.", which is far below the temp you'll be storing beer at (35-55ºF). And to top it off, it states that "When fully cured, J-B Weld is completely resistant to water, gasoline, and about every other petroleum product or automotive chemical." So it will not just dissolve into your beer.
I now point you to a post I found right here on HBT: http://www.homebrewtalk.com/f51/will-exposed-jb-weld-cause-problems-hlt-195002/#post2266853
This post has a link to an NSF certified test of several JB Weld products in contact with a potable water system. It states that "The exposed surface area of each sealant in a household distribution system shall not exceed 1.3 sq. cm/L or .2 sq. in./L." So let's do some simple math.
5 gallons = 19 Liters and 19 L * 0.2 sq. in./L = 3.8 sq. in. or 24.7 sq. cm for the non-Americans out there.
This tells me that unless you decide to coat 3.8 sq. inches of the inside surface of your corny keg with JB Weld and it's in constant contact with your beer, you'll be completely fine.
Lastly, we turn to the actual MSDS of JB Weld. If you look at the ingredients list many of them are food safe in and of themselves. The ones in question are all either solvents (they evaporate completely upon full cure) or active ingredients (they are rendered inert by the curing reaction). So there you have it. JB Weld is fine to use for sealing a small 1/4" hole with a level sensor at the top of the keg that is unexposed to beer. Of course, you can always choose to use something else, but that's all your own preference.